The present embodiments relate to single photon emission computed tomography (SPECT). In particular, the present embodiments relate to dead time compensation in SPECT.
During SPECT imaging, the detector electronics take time to perform detection of an emission. During this period, additional emissions are not detected due to the unavailability of the detector electronics. As a result, the actual emissions may be under counted. The count of detected emissions is corrected for system dead time. In one approach, a signal is input at fixed frequency and amplitude to the detector electronics. Due to the fixed frequency, a known number of signals is input. Due to dead time from detecting emissions from the patient, some of the fixed frequency signals are not detected. The ratio of detected ones of the fixed frequency to the input number provides a measure of dead time. The count of detected emissions from the patient is divided by the ratio to correct for the dead time.
Since the fixed frequency signal is input at the electronics, any contribution of the detection system before signal processing to the dead time is ignored. This contributes to uncertainty in the reconstructed image. In quantitative SPECT, an inexact correction may result in an inexact quantification. When imaging therapy isotopes with corresponding high-count rates, the inaccuracy may be more significant. These approaches also require that the processing is independent of the pulse height, which may not be the case. High count rates during imaging may lead to spectral broadening, which also results in undercounting in the preset energy window due to change in resolution.